1. Field of the Invention
The present invention relates to a Ni—Al alloy anode for molten carbonate fuel cell made by in-situ sintering the Ni—Al alloy and a method for making the Ni—Al alloy anode.
2. Description of the Related Art
A fuel cell is used for directly converting chemical energy into electric energy. There are various kinds of the fuel cell, such as a molten carbonate fuel cell, a solid polymer electrolyte fuel cell, and a solid oxide fuel cell etc. The molten carbonate fuel cell is a fuel cell using the molten carbonate as its electrolyte and comprises a cathode, an electrolyte, a support and an anode etc.
A high temperature fuel cell operating at 500° C. or more, such as the molten carbonate fuel cell and the solid oxide fuel cell, mainly uses nickel for an electrode. For example, the molten carbonate fuel cell uses nickel for an anode and nickel oxide (NiO) for a cathode.
The anode in which an oxidation reaction of fuel occurs has serious problems of sintering and creep phenomena at an operating condition of high temperature and high load of 2 kg/cm2 or more. That is, the reduction of porosity and the change of micro-structure such as a shrinkage etc. occur in the anode due to the sintering and the creep phenomena, thereby causing a degradation of the performance of the fuel cell.
In particular, the nickel electrode is manufactured to be porous so as to enlarge a reaction area and to provide a gas passage. When such a porous nickel electrode is used at a high temperature for a long time, the surface area and reaction rate of the nickel electrode are reduced due to the sintering. In addition, when a fuel cell having stacks of several unit cells using the porous nickel electrode is operated for a long time, there occurs a creep in the porous nickel electrode due to the load of the fuel cell, thereby reducing the performance of the fuel cell.
In the prior art for solving the above-mentioned problems of the sintering and the creep, a chromium of about 10 wt % was added to the nickel or an oxide such as Cr2O3 and LiCrO2 was formed on the surface of the nickel electrode in order to improve the resistance of the nickel electrode to the sintering and the creep. It is known that a creep strain of Ni+10% Cr anode is 5% or less. However, the LiCrO2 formed on the surface of the nickel electrode is dissolved in the electrolyte, thereby deteriorating the resistance of the nickel electrode to the sintering and the creep when the fuel cell is operated for a long time.
In order to improve the creep characteristic, there has been used an oxide dispersion strengthened (ODS) method of dispersing a metal oxide such as alumina in the nickel electrode since the mid-1980's. Further, there have been extensive studies for an electrode consisting of Ni—Al based alloy containing a small amount of aluminum, which is oxidized prior to nickel. It is known that the electrode consisting of Ni—Al based alloy has a creep strain of 0.5% or less and an increase of contact resistance is very slight even in a size of 1 m2, which is a size of a commercial electrode.
However, the prior Ni—Al alloy electrode is expensive compared to the electrode using the existing material and has such a problem that the Ni—Al alloy electrode is not sintered in a general manufacturing process of the electrode. That is, the aluminum formed as a solid solution is primarily oxidized on the surface of the nickel electrode, thereby forming an alumina oxide having a very high melting point on the surface. Due to the alumina oxide, the sintering between the nickel particles becomes difficult.
Under the circumstances, there has been used a method of sintering the electrode through a partial oxidation-reduction wherein the surface of the electrode is partially oxidized under the condition that the nickel particle can be oxidized, and then the surface of the electrode is reduced again.
The partial oxidation-reduction method uses a phenomenon that when the nickel is oxidized into a nickel oxide, the density is changed and a volume is thus expanded. According to the partial oxidation-reduction method, the nickel particles can easily contact with each other due to the surface oxidation of the nickel, so that it is possible to progress a sintering process despite the formation of the alumina oxide and to manufacture an electrode having a proper strength. In the partial oxidation-reduction method, it is very important to control a partial pressure of oxygen, thereby preventing an occurrence of excessive micro-pores due to the volume change between nickel and nickel oxide resulting from the excessive oxidation of the nickel particle. The micro-pores eventually cause a re-distribution of the electrolyte, thereby exerting very bad influence on the life of the fuel cell.
As mentioned above, since the partial pressure of oxygen should be controlled, there have been such problems in the partial oxidation-reduction method that it is very difficult to introduce a continuous process required for a mass production and that necessary equipments become very complicated.